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After the campaign? Impasse. If the opinion polls are half-right David Cameron’s Conservatives and Ed Miliband’s Labour are staring into the barrel of shared defeat. Whether it is a Tory government propped up by Liberal Democrats or a Labour one reliant on Scottish nationalists, the organising fact of Britain’s political landscape will be that the prime minister lacks a mandate. History will remember the election for the spectacular charge of the Scottish National party.

The electorate has rejected the Tory prescription of an ever-shrinking state. More than half, the pollsters say, want a change of economic direction. The Conservatives surrendered last autumn any residual hope of winning outright when George Osborne, the chancellor, trumpeted five more years of austerity. Britain’s voters like prudence. They disdain ideology. To adapt a phrase, it was the Treasury what lost it for Mr Cameron.

Time for a change does not translate into enthusiasm for Mr Miliband. The Labour leader has outperformed expectations that could hardly have been set lower. But the Britain that wants to look beyond the horizon of harsh spending cuts also harbours serious doubts that Labour would spend wisely. No one likes bankers, but Mr Miliband’s business-bashing has driven him into his own ideological cul-de-sac.

The deep truth behind all this is that the two big parties are broken. The Tory and Labour tribes have shrunk and, along the way, fallen under the spell of doctrinal zealots. Power in Mr Cameron’s party has shifted towards elderly activists who hanker for the world as they imagine it once was. Labour is beholden for its funding to a handful of trade unions that never quite got over the fall of the Berlin Wall. In both cases, the impulse is to turn inwards.

The two leaders have paid a price for ignoring the uncommitted. The English nationalists of the UK Independence party and Scotland’s SNP have exploited the economic insecurities of the age. When times are hard identity politics cast a powerful spell — witness the rise of populist parties of left and right across Europe. But the nationalists have also prospered from the absence of a serious attempt to wrench the argument back to a middle ground.

Sure, voters are worried about globalisation, low pay and the sometimes heavy hand of Brussels. They want a well-policed welfare state and an immigration system that is fair and secure. And, yes, they resent the way the super rich have scooped up the gains of globalisation. But most are where they have always been: in favour of an open market economy as long as it sits alongside a decent state safety net.

In most other European nations the answer to the present deadlock would be a grand coalition to mobilise the middle. Faced with such a stalemate an intelligent prime minister would reach beyond the arithmetic of a precarious majority or minority government and head for this hard centre of politics. It should not be all that difficult. Such a government would propose to cut the deficit without making it a fetish, to look for a better deal from the EU but know Britain cannot go it alone, and, above all, to shape a constitutional settlement to reconcile Scottish demands for home rule with the preservation of the fabric of the four-nation union.

Sadly, such a realignment is beyond the possible for now, unless Britain can borrow Germany’s Angela Merkel. Mr Cameron and Mr Miliband will cling on to the ancien regime. It will probably take another inconclusive election and a new set of leaders to properly understand that, along with their parties, the postwar political mould is truly broken.

Scotland, though, cannot wait. The surge in support for the SNP has not overturned the outcome of last September’s independence referendum. The first-past-the-post electoral system translates the 45 per cent share of the vote the nationalists won in the referendum to victory in up to 50 of the 59 Scottish seats at Westminster. That said, the nationalists remain an existential threat to the union.

You would not have grasped this from the tenor of the campaign. For Mr Cameron, the nationalists have been a stick with which to beat Labour — vote for Mr Miliband and SNP leader Nicola Sturgeon will pull the strings. The Labour riposte was that a vote for the SNP promised to put Mr Cameron back in Downing Street. The shared message has been that the party elected to rule Scotland was unfit for any role in the governance of the UK. The two leaders could scarcely have offered a better route map for the break-up of Britain.

There can be no more playing the English card unless the Tories want the UK to go the way of the two-party system. The choice is between partisanship and the union. Last autumn, the parties put aside their differences to offer Edinburgh more devolution in a rebalanced UK. Scots now expect Westminster — whatever its political complexion — to keep its side of the bargain.

This will not be easy. A union of such unequal states does not lend itself to neat federalist blueprints. Fairness demands decentralisation in England as well as devolution elsewhere. The tax and spending implications are fiendishly complicated. All this will require open-minded cross-party negotiation. The politicians should remind themselves that Britain has been here before. In 1910 the Westminster parties bungled Irish Home Rule. We know what happened next.